Typically, as a structure of connecting an electrode of a semiconductor element with one or more inner leads, bonding of a wire consisting of conductive material such as gold is well known.
The wire bonding connection structure is shown in FIG. 7. FIG. 7(a) is a partially enlarged plan view and FIG. 7(b) is a partially enlarged front view of the conventional wire bonding structure. In this structure, a ball 7-2 is formed at one end of the wire 7-1 consisting of conductive material such as gold or aluminum, and is bonded to the electrode (bonding pad) 5 provided on the semiconductor element 4. The other end of the wire 7-1 is connected to the inner lead 3 with a wedge bonding technique, thereby, the bonding pad 5 on the semiconductor element 4 and the inner lead 3 are electrically connected to each other.
However, the above described conventional structure has several problems.
First, the number of inner leads increases and the space D1 between the inner leads of the lead frame shaped by etching or punching becomes very narrow, as the semiconductor elements are highly integrated and involve multiple functions. As is already known, the size of the space D1 between the inner leads 3 is defined by the thickness of the metal sheet used for a lead frame, and the minimum distance is limited to this thickness. Accordingly, the distance D2 between the bonding pad 5 and the inner lead 3 inevitably becomes long, which means lengthening of the wire 7-1. When the wire 7-1 becomes long, it is caught in the flow of resin which is injected at high pressure during the molding process. This causes a short circuit by touching with other wires or with the semiconductor element 4. In order to avoid this, the wire must be shortened. However, since it is impossible to bring the inner lead 3 close to the semiconductor element 4, the bonding pad 5 must be brought close to the inner lead 3. This results in an increase of the area of the semiconductor element 4 to an area larger than that necessary for serving its functions, and the number of the semiconductor element per wafer decreases and the cost rises.
Secondly, cross connection of the wire 7-1 is not used because it is apt to cause a short circuit by contact of the wire 7-1.
Thirdly, in a wire bonding connection, one end of the wire is always connected by wedge bonding, which needs a larger contact area than ball bonding because the edge of the capillary is pressed onto the connected surface. Thus, when directly connecting two bonding pads 5 on a single semiconductor element 4 or connecting two semiconductor elements 4 with wire, the area of the semiconductor element 4, as well as the area of the bonding pad 5 for wedge bonding, become large. So such a direct connection between the bonding pad 5 is not employed in reality.
As another connecting structure, there is the TAB (Tape Automated Bonding) method which meets with the high integration and multi-pin structure of a semiconductor device. This method, however, has not come into wide use because it requires a dedicated mounting apparatus since a film carrier itself is mounted onto the substrate.